The fastener types required for joining composite materials must meet criteria that are quite different from those fasteners which are acceptable for joining homogeneous metallic materials. Composite materials are generally constructed of high-strength fibers, sometimes of carbon or graphite material for light weight, which are contained in a resin base for form and rigidity. Epoxy resins are commonly used, but others are also useful. During the installation of a fastener it is essential that the strength of the laminate composite is not exceeded, in particular with respect to the bond between the fiber lamination and the resin binders. If the ultimate strength of this cohesion is exceeded, then the material will be delaminated in the area of the fastener and can lead to early corrosion and fatigue failure of the joint. Therefore, it is essential that the installation pressures be minimized, but for economic reasons it is desirable to be able to produce a hole with economically practical tolerance allowances for its inside diameter.
It is also necessary to produce a fastener to fit in the laminar material hole that will be close enough to prevent excessive working of the hole under stress by movement of the joint. Such movement can produce very deleterious high localized stress concentrations.
A further risk to be avoided in the installation of fasteners in composite materials is the inducement of such high radial stresses as may produce delamination of the material.